Inclusion Body Myositis is an inflammatory myopathy whose precise pathogenesis is unknown. It has not effective treatment. This study hypothesizes that immune-mediated muscle damage is of primary or secondary importance in producing the muscle weakness and wasting that characterizes IBM. Although corticosteroids and other immune suppressant medications may be of minor benefit in some patients, they are generally effective. Immunomodulating agents, notably the interferons, appear to hold promise in a number of inflammatory diseases. Beta interferon has been approved for use in multiple sclerosis, a disease with features suggesting a still-undefined immune pathogenesis and for which corticosteroids are of limited benefit. This study will investigate the tolerability and safety of intramuscular Beta interferon and will indentify problems with and refine the various muscle testing procedures to optimize their usefulness as primary outcome measures.